1. Field of the Invention
The invention relates to improvements in the manufacture of sulfhydryl-terminated compounds useful as chemical intermediates and precursors of polythioether elastomeric sealants. By the method of the invention, products of improved purity are obtained by the addition of hydrogen sulfide to an allene compound. Subsequent addition of the sulfhydryl compound to a polyunsaturated olefin and/or an acetylene produces sulfhydryl-terminated polythioethers of improved purity and color. Such polythioethers are useful as a base polymer for sealants.
2. Description of the Prior Art
The ultraviolet light initiated free radical addition of hydrogen sulfide to allene in the liquid phase is known to produce sulfhydryl-terminated compounds such as mercaptans and sulfhydryl-terminated thioethers. U.S. Pat. No. 3,488,270 to Griesbaum et al. discloses the liquid phase addition of hydrogen sulfide to allene initiated with ultraviolet light. The reaction is conducted in quartz which transmits high energy ultraviolet light, i.e. ultraviolet light of wave length below 2600 Angstroms as well as low energy, long wavelength ultraviolet light, i.e. ultraviolet light of wavelength above about 2600 Angstroms. The reaction product is yellow and is usually contaminated with elemental sulfur. The formation of sulfur is accompanied by hydrogen evolution which may produce sufficient pressure to burst the reaction vessel. It has not been found feasible by conventional purification techniques to remove the discoloration. This disadvantage is particularly serious since the principal reaction product, 1,3-propane dithiol, is the precursor of sulfhydryl-terminated polythioethers which cure to valuable elastomeric sealants. Thus when the discolored dithiol, is reacted with an acetylenically and/or polyethylenically unsaturated compound in the presence of ultraviolet light as initiator according to known techniques (see U.S. Pat. Nos. 3,592,798 and 3,717,618 to Oswald and coassigned U.S. Ser. No. 501,716, filed Aug. 29, 1974, entitled "Polythioether Sealant Compositions" to B. F. Dannels), the sulfhydryl polythioether obtained is also discolored and of unsatisfactory curing characteristics and, hence is commercially unacceptable.
U.S. Pat. Nos. 2,398,481 and 2,411,983 to W. E. Vaughan et al. and U.S. Pat. No. 2,873,239 to W. R. Nummy et al. disclose the liquid phase addition of hydrogen sulfide to unsaturates initiated by ultraviolet light of wavelength above 2900-3000 Angstroms. The references neither relate to use of reactants having the distinctive allene unsaturated group, i.e. two carbon to carbon double bonds attached to the same carbon atom, nor teaches substantial conversion to sulfhydryl-terminated products in absence of a photosensitizing reagent.
U.S. Pat. No. 3,412,001 to J. R. Edwards discloses liquid phase reaction of hydrogen sulfide and olefins in the presence of ultraviolet light filtered through Pyrex, i.e. ultraviolet light of wavelength substantially above about 2800-2900 Angstroms. However, the patent does not relate to reaction of olefins containing the distinctive allene linkage and, moreover, requires limiting the reaction to conversion of no more than 22% of olefin starting material.